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.TH PTHREAD_MUTEX_LOCK 3C "Nov 11, 2008"
.SH NAME
pthread_mutex_lock, pthread_mutex_trylock, pthread_mutex_unlock \- lock or
unlock a mutex
.SH SYNOPSIS
.LP
.nf
cc -mt [ \fIflag\fR... ] \fIfile\fR... -lpthread [ \fIlibrary\fR... ]
#include <pthread.h>

\fBint\fR \fBpthread_mutex_lock\fR(\fBpthread_mutex_t *\fR\fImutex\fR);
.fi

.LP
.nf
\fBint\fR \fBpthread_mutex_trylock\fR(\fBpthread_mutex_t *\fR\fImutex\fR);
.fi

.LP
.nf
\fBint\fR \fBpthread_mutex_unlock\fR(\fBpthread_mutex_t *\fR\fImutex\fR);
.fi

.SH DESCRIPTION
.sp
.LP
The mutex object referenced by mutex is locked by calling
\fBpthread_mutex_lock()\fR. If the mutex is already locked, the calling thread
blocks until the mutex becomes available. This operation returns with the mutex
object referenced by mutex in the locked state with the calling thread as its
owner.
.sp
.LP
If the mutex type is  \fBPTHREAD_MUTEX_NORMAL\fR, deadlock detection is not
provided. Attempting to relock the mutex causes deadlock. If a thread attempts
to unlock a mutex that it has not locked or a mutex that is unlocked, undefined
behavior results.
.sp
.LP
If the mutex type is  \fBPTHREAD_MUTEX_ERRORCHECK\fR, then error checking is
provided. If a thread attempts to relock a mutex that it has already locked, an
error will be returned. If a thread attempts to unlock a mutex that it has not
locked or a mutex which is unlocked, an error will be returned.
.sp
.LP
If the mutex type is  \fBPTHREAD_MUTEX_RECURSIVE\fR, then the mutex  maintains
the concept of a lock count. When a thread successfully acquires a mutex for
the first time, the lock count is set to 1. Every time a thread relocks this
mutex, the lock count is  incremented by one. Each time the thread unlocks the
mutex, the lock count is decremented by one. When the lock count reaches
\fB0\fR, the mutex becomes available for other threads to acquire. If a thread
attempts to unlock a mutex that it has not locked or a mutex that is unlocked,
an error will be returned.
.sp
.LP
If the mutex type is  \fBPTHREAD_MUTEX_DEFAULT\fR, attempting to recursively
lock the mutex results in undefined behavior. Attempting to  unlock the mutex
if it was not locked by the calling thread results in undefined behavior.
Attempting to unlock the mutex if it is not locked results in undefined
behavior.
.sp
.LP
The \fBpthread_mutex_trylock()\fR function is identical to
\fBpthread_mutex_lock()\fR except that if the mutex object referenced by
\fImutex\fR is currently locked (by any thread, including the current thread),
the call fails immediately with \fBEBUSY\fR.
.sp
.LP
The \fBpthread_mutex_unlock()\fR function releases the mutex object referenced
by \fImutex\fR. The manner in which a mutex is released is dependent upon the
mutex's type attribute. If there are threads blocked on the mutex object
referenced by \fImutex\fR when \fBpthread_mutex_unlock()\fR is called,
resulting in the mutex  becoming available, the scheduling policy is used to
determine  which thread will acquire the mutex. (In the case of
\fBPTHREAD_MUTEX_RECURSIVE\fR mutexes, the mutex becomes available when the
count reaches  \fB0\fR and the calling thread no longer has any locks on this
mutex.)
.sp
.LP
If a signal is delivered to a thread waiting for a mutex, upon return from the
signal handler the thread resumes waiting for the mutex as if it was not
interrupted.
.SH RETURN VALUES
.sp
.LP
If successful, the  \fBpthread_mutex_lock()\fR and
\fBpthread_mutex_unlock()\fR functions return  \fB0\fR. Otherwise, an error
number is returned to indicate the error.
.sp
.LP
The  \fBpthread_mutex_trylock()\fR function returns  \fB0\fR if a lock on the
mutex object referenced by \fImutex\fR is acquired. Otherwise, an error number
is returned to indicate the error.
.SH ERRORS
.sp
.LP
The  \fBpthread_mutex_lock()\fR and \fBpthread_mutex_trylock()\fR functions
will fail if:
.sp
.ne 2
.na
\fB\fBEAGAIN\fR\fR
.ad
.RS 10n
The mutex could not be acquired because the maximum number of recursive locks
for mutex has been exceeded.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The \fImutex\fR was created with the protocol attribute having the value
\fBPTHREAD_PRIO_PROTECT\fR and the calling thread's priority is higher than the
mutex's current priority ceiling.
.RE

.sp
.ne 2
.na
\fB\fBEPERM\fR\fR
.ad
.RS 10n
The mutex was created with the protocol attribute having the value
\fBPTHREAD_PRIO_PROTECT\fR and the calling thread is not in the real-time class
(\fBSCHED_RR\fR or \fBSCHED_FIFO\fR scheduling class).
.RE

.sp
.LP
The  \fBpthread_mutex_trylock()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEBUSY\fR\fR
.ad
.RS 9n
The \fImutex\fR could not be acquired because it was already locked.
.RE

.sp
.LP
The  \fBpthread_mutex_lock()\fR, \fBpthread_mutex_trylock()\fR and
\fBpthread_mutex_unlock()\fR functions may fail if:
.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The value specified by \fImutex\fR does not refer to an initialized mutex
object.
.RE

.sp
.LP
The  \fBpthread_mutex_lock()\fR function may fail if:
.sp
.ne 2
.na
\fB\fBEDEADLK\fR\fR
.ad
.RS 11n
The current thread already owns the mutex.
.RE

.sp
.ne 2
.na
\fB\fBENOMEM\fR\fR
.ad
.RS 11n
The limit on the number of simultaneously held mutexes has been exceeded.
.RE

.sp
.LP
The  \fBpthread_mutex_unlock()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEPERM\fR\fR
.ad
.RS 9n
The mutex type is PTHREAD_MUTEX_ERRORCHECK or the mutex is a robust mutex, and
the current thread does not own the mutex.
.RE

.sp
.LP
When a thread makes a call to \fBpthread_mutex_lock()\fR or
\fBpthread_mutex_trylock()\fR, if the mutex is initialized with the robustness
attribute having the value \fBPTHREAD_MUTEX_ROBUST\fR (see
\fBpthread_mutexattr_getrobust\fR(3C)), the call will return these error values
if:
.sp
.ne 2
.na
\fB\fBEOWNERDEAD\fR\fR
.ad
.RS 19n
The last owner of this mutex died while holding the mutex, or the process
containing the owner of the mutex unmapped the memory containing the mutex or
performed one of the \fBexec\fR(2) functions. This mutex is now owned by the
caller. The caller must now attempt to make the state protected by the mutex
consistent. If it is able to clean up the state, then it should call
\fBpthread_mutex_consistent()\fR for the mutex and unlock the mutex. Subsequent
calls to \fBpthread_mutex_lock()\fR and \fBpthread_mutex_trylock()\fR will
behave normally, as before. If the caller is not able to clean up the state,
\fBpthread_mutex_consistent()\fR should not be called for the mutex, but the
mutex should be unlocked. Subsequent calls to \fBpthread_mutex_lock()\fR and
\fBpthread_mutex_trylock()\fR will fail to acquire the mutex with the error
value \fBENOTRECOVERABLE\fR. If the owner who acquired the lock with
\fBEOWNERDEAD\fR dies, the next owner will acquire the lock with
\fBEOWNERDEAD\fR.
.RE

.sp
.ne 2
.na
\fB\fBENOTRECOVERABLE\fR\fR
.ad
.RS 19n
The mutex trying to be acquired was protecting the state that has been left
irrecoverable by the mutex's last owner. The mutex has not been acquired. This
condition can occur when the lock was previously acquired with
\fBEOWNERDEAD\fR, and the owner was not able to clean up the state and unlocked
the mutex without calling \fBpthread_mutex_consistent()\fR.
.RE

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Committed
_
MT-Level	MT-Safe
_
Standard	See \fBstandards\fR(7).
.TE

.SH SEE ALSO
.sp
.LP
.BR pthread_mutex_consistent (3C),
.BR pthread_mutex_init (3C),
.BR pthread_mutexattr_setprotocol (3C),
.BR pthread_mutexattr_setrobust (3C),
.BR pthread_mutexattr_settype (3C),
.BR attributes (7),
.BR standards (7)
.SH NOTES
.sp
.LP
In the current implementation of threads, \fBpthread_mutex_lock()\fR,
\fBpthread_mutex_unlock()\fR, \fBmutex_lock()\fR, \fBmutex_unlock()\fR,
\fBpthread_mutex_trylock()\fR, and \fBmutex_trylock()\fR do not validate the
mutex type.  Therefore, an uninitialized mutex or a mutex with an invalid type
does not return  \fBEINVAL\fR. Interfaces for mutexes with an invalid type have
unspecified behavior.
.sp
.LP
Uninitialized mutexes that are allocated locally may contain junk data. Such
mutexes need to be initialized using  \fBpthread_mutex_init()\fR or
\fBmutex_init()\fR.
